The present invention relates to a laser beam projection mask, a laser beam machining method and a laser beam machine used to crystallize amorphous materials, which are for use, for example, as semiconductor materials in semiconductor devices and the like, through laser beam irradiation.
While there is a method using single crystal silicon (Si) as a general manufacturing method for semiconductor devices, there is another manufacturing method, other than the manufacturing method, which uses silicon thin films produced by forming silicon thin films on glass substrates. Semiconductor devices manufactured with use of the silicon thin films formed on the glass substrates are used as part of image sensors and active-matrix liquid crystal displays.
Herein, in the liquid crystal display, the semiconductor device is used as a TFT (Thin Film Transistor) arranged as a regular array on a transparent substrate. Each transistor in the TFT functions as a pixel controller in the liquid crystal displays. It is to be noted that the TFTs in the liquid crystal displays are conventionally formed from amorphous silicon films.
However, in recent years, polycrystalline silicon films with high electron mobility is being used instead of amorphous silicon films with low electron mobility to manufacture TFT liquid-crystal displays having a strengthened TFT switching characteristic and an increased display speed. Herein, methods for manufacturing polycrystalline silicon films include, for example, a (ELC: Excimer Laser Crystallization) method to crystallize amorphous or microcrystalline silicon films deposited on substrates through irradiation of excimer laser beams.
The ELC method is generally composed of scanning samples at a constant rate while continuously irradiating a semiconductor film with a linear laser beam having a length of 200 to 400 mm and a width of 0.2 to 1.0 mm. In this case, a portion of the semiconductor film irradiated with the laser beam does not melt entirely along the thickness direction but melts while a part of the semiconductor film region remains.
Consequently, on the entire interface between a unmelted region and a melted region, crystalline nucleuses are produced at every place and crystals grow toward the uppermost layer of the semiconductor film, so that crystal grains with random orientation are formed and their crystal grain size becomes as small as 100 to 200 nm.
In the grain boundaries in polycrystalline silicon film, a number of unpaired electrons are present and form potential barriers so as to function as strong scatterers of carriers. Therefore, the TFTs formed from polycrystalline silicon films with a smaller number of grain boundaries, i.e., with a larger crystal grain size, generally have higher electron field-effect mobility.
However, as described above, the conventional ELC method involves vertical crystal growth in which crystallization occurs at random positions on the boundaries between the unmelt region and the melt region, which makes it difficult to obtain polycrystalline silicon films with a large grain size. Because of this reason, it was difficult to obtain TFTs with high electron field-effect mobility. Moreover, since crystallization occurs at random, such defects as structural heterogeneity among respective TFTs and heterogeneity in the switching characteristic of TFT arrays are generated. Moreover, when these defects are generated, the TFT liquid crystal displays suffer the problem that pixels with a high display speed and pixels with a low display speed are present in one display screen.
Accordingly, for obtaining TFT liquid crystal displays with higher performance, it is necessary to increase the crystal grain size of the polycrystalline silicon films and to control the orientation of silicon crystals. Eventually, for the purpose of obtaining polycrystalline silicon films having capability close to monocrystal silicon, a number of proposal has been made.
Among these proposals, the laser crystallization technique classified as “lateral growth method” in particular is attracting a lot of attentions since it can provide long crystals whose orientation is aligned in the growth direction of crystals (e.g., Japanese translation of PCT international application No. 2000-505241).
The method is to crystallize a semiconductor by irradiating the semiconductor with a pulse laser beam having a minute width by a laser beam machine as shown in FIG. 14 so as to melt and solidify a semiconductor film across the entire thickness-direction region of a laser irradiation region. According to the laser beam machine, an excimer laser beam emitted from a light source 211 travels through a variable attenuator 212, a varifocal field lens 213, a projection mask 214 and an image-forming lens 215 before being irradiated onto the upper surface of a semiconductor element 101. The semiconductor element 101 includes a transparent substrate having optical transparency. Herein, as shown in FIG. 12, the semiconductor element 101 is composed of a transparent substrate 102, a base film 103 formed on the transparent substrate 102 and a silicon film 104.
The steps in the method are shown below. First, as shown in FIG. 12, for forming a crystal region along an extending direction (AB direction as viewed in the drawing) of the semiconductor film 104 on the transparent substrate 102, heat is induced to a region C in the semiconductor film 104. The induction of heat is performed by exposing the semiconductor film 104 to a laser beam after the region other than the region C on the semiconductor film 104 is masked. By this, energy of a laser beam 105 irradiated onto the region C is converted to thermal energy so that heat can be induced to the region C in the semiconductor film 104 and the region C can be melted over the thickness of the semiconductor film 104.
Next, when the semiconductor film 104 melted in the region C is solidified by cooling, crystals grow from boundaries C1 and C2 between the region C and other regions toward the center of the region C as shown in FIG. 13A. It is to be noted that FIG. 13A is a top view of the semiconductor film 104 in FIG. 12.
Further, as shown in FIG. 13B, a new region D adjacent to the region C is so set as to include a portion where crystals are not formed in the region C, and is melted in the similar way as in the aforementioned steps. Then, when the semiconductor film 104 which were melt in the region D is solidified in a similar way, crystals grow in the region D as shown in FIG. 13C. By forming desired crystals along the extending direction of the semiconductor film 104 gradually by repeating such steps, semiconductor crystals of polycrystalline structure can be expanded as shown in FIG. 13D. This makes it possible to form a polycrystalline silicon film with a large crystal grain size.
By the way, energy of the leaser beam 105 incident into the semiconductor film 104 is ideally homogenized at positions with respect to a specified direction as shown in FIG. 15A. More particularly, the laser beam is deformed to have a homogeneous beam shape by a homogeneous optical system and is incident into the semiconductor film 104 as the leaser beam 105. Moreover, as shown in FIG. 15B, three slits 225 formed in the projection mask 214 are disposed at specified intervals, the respective slits 225 having a constant width and being formed almost equally with respect to a specified direction. Moreover, in another projection mask 231 shown in FIG. 15C, the shape of respective slits 232 (slit width in particular) is identical, and the respective slits 232 are disposed almost equally in a specified direction.
By irradiating the semiconductor film 104 with the leaser beam 105 which transmitted the slits 225, 232 in the projection masks 214, 231, a uniform crystal grain length can be obtained, which allows the semiconductor element 101 to move at an identical speed or move stepwise with an identical width.
However, the aforementioned prior art has a following problem. That is, the laser beam is designed to come incident in the state of being deformed to have a homogeneous beam shape by a homogeneous optical system for the purpose of obtaining a uniform crystal grain length.
However, parameters concerning crystallization itself regard heat generated by a laser beam. Therefore, when the semiconductor element 101 is scanned by a laser beam at a high speed for expanding a crystallized region, a laser beam distribution is not necessarily congruous with a heat distribution on the surface of the semiconductor film 104, which causes the problem that a uniform crystal grain length cannot be obtained.
In the case where such a uniform crystal grain length cannot be obtained, the slit width needs to be set in conformity with the minimum value of the crystal grain length for achieving seamless expansion of the crystallized region. However, when the crystal grain length is increased with the slit having such width the problem of deteriorated crystallinity occurs.
Further, although in the prior art, a laser beam is deformed to have a homogeneous beam shape by a homogeneous optical system, complete homogenization of the laser beam causes such problems as difficulty in adjustment associated with increase in the number of optical components and degradation in efficiency. Moreover, while there are cases of using first and second two laser beams for efficiently increasing the temperature of substrates and semiconductor materials, there is a problem that normal masks cannot cope with both the first laser beam and the second laser beam.